Switching power supplies using discrete components are usually constructed with at least two transistors. One transistor is used for switching current in a primary winding of a transformer coupled in series with a main current path of the transistor. The other one transistor is used for providing over-current protection in the first transistor. It may be desirable to form a switching power supply with a power switching Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) as an active element such that, without any additional active element, both self-oscillation and over-current protection are provided by the MOSFET.
In carrying out an inventive feature, a transformer that is included in a positive feedback path of the MOSFET has a tapped auto-transformer winding. A source terminal of the MOSFET is coupled via a current limiting or sampling resistor to a junction terminal between first and second windings of the tapped auto-transformer winding. The first winding forms the primary winding of the transformer and the second winding is coupled to a gate terminal of the MOSFET to form a regenerative feedback path.
In carrying out another inventive feature, the second winding is direct-current (DC) coupled to the gate terminal to avoid the need for any discrete capacitor in the positive feedback path. Thereby, advantageously, the power supply is simplified.